CMake explicit language

CMake 3.1 is used in main CMakeLists file, no reason not to use it in example.
Threads::Threads is introduced in cmake 3.1.

Marking project as CXX will disable detecting C compiler and other checks.
Removed 'INCLUDES DESTINATION' as it made the include path appear doubled.

Fix compilation error with GCC 8

error: need 'typename' before 'std::conditional<std::is_same<char, char>::value, fmt::BasicMemoryWriter<char>, fmt::BasicMemoryWriter<wchar_t> >::type' because 'std::conditional<std::is_same<char, char>::value, fmt::BasicMemoryWriter<char>, fmt::BasicMemoryWriter<wchar_t> >' is a dependent scope
         std::conditional<std::is_same<filename_t::value_type, char>::value, fmt::MemoryWriter, fmt::WMemoryWriter>::type w;

Make set_color public in wincolor_sink to retain configurability

Modern CMake in /tests

Fixed spelling mistake in example.cpp

A few small ANSI color improvements

Use Sleep in Windows instead of sleep_for

fixes #609

Ensure marcos always expand to expressions

Add global flush_on function

Issue fix for spdlog #595. Conversion warning.

See: https://github.com/gabime/spdlog/issues/595

On line 85 in file sinks/wincolor_sink.h:
back_color &= ~(FOREGROUND_RED | FOREGROUND_GREEN | FOREGROUND_BLUE |
FOREGROUND_INTENSITY);

'back_color' is of type 'WORD' (unsigned short) whereas a bitwise
complement/NOT returns an int. This results in a conversion warning with
-Wconversion enabled.

85:20: warning: conversion to 'WORD {aka short unsigned int}' from 'int'
may alter its value [-Wconversion] back_color &= ~(FOREGROUND_RED |
FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY);

Possible solution:
We know that the result of ~(FOREGROUND_RED | FOREGROUND_GREEN |
FOREGROUND_BLUE | FOREGROUND_INTENSITY) is always within the limits of
an unsigned short so a simple cast should suffice (correct me if I'm
wrong):

back_color &= static_cast<unsigned short>(~(FOREGROUND_RED |
FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY));